1. Field of the Invention
This invention relates to an optical subscriber network system suitable for use with a passive optical network system and a fault supervising method for an optical subscriber network system.
2. Description of the Related Art
FIG. 12 schematically shows a construction of an ordinary station subscriber network. In the construction of the ordinary station subscriber network shown in FIG. 12, for optincal network units (ONU) 1' to N' (N' is a natural number) which require high speed communication of a large capacity, optical fibers are usually laid in a one by one corresponding relationship from a station apparatus 100' to accommodate the optincal network units (ONU) 1' to N' in the station.
In recent years, as a system which economically accommodates subscribers who require broad-band communication, such a passive optical network (PON) system as shown in FIG. 13 has been proposed taking the economy and so forth into consideration.
In the passive optical network (PON) system shown in FIG. 13, an optical distributor-coupler (star coupler) 50' formed from a passive element is interposed between a station apparatus 100" and optical network units (ONU) 1" to N" (N" is a natural number), and one pair of optical fibers or, for the object of redundancy, two pairs of optical fibers, are laid between the station apparatus 100' and the optical distributor-coupler 50'. The optical distributor-coupler 50' distributes downstream optical signals from the station apparatus 100' to the optical network units (ONU) 1" to N". On the other hand, optical signals from the optical network units (ONU) 1" to N" are transmitted in upstream frames only for times of time slots allocated thereto and then coupled by the optical distributor-coupler 50', whereafter they are transmitted to the station apparatus 100".
Here, for upstream communication from the optical network units (ONU) 1" to N" to the station apparatus 100". TDMA (Time Division Multiple Access) is used, and for downstream communication, TCM (Time Compression Multiplexing) which is a combination of TDM (Time Division Multiplexing) is used.
A construction of a frame of the TDM/TDMA described above is illustrated in FIG. 14.
Accordingly, with the passive optical network (PON) system, since the optical transmission lines and the subscriber optical transmission lines of the station apparatus 100" are used commonly, the cost required for construction of the system can be reduced comparing with the station subscriber network construction shown in FIG. 12 wherein the station apparatus 100' and the optincal network units (ONU) 1' to N' are connected in a one by one corresponding relationship to each other.
Further, since a passive element is used as an optical distributor-coupler, improvement in reliability of the system can be anticipated comparing with another system wherein optical signals are multiplexed and demultiplexed using an active apparatus.
However, with the construction of the passive optical network (PON) shown in FIG. 13, while dual construction of the transmission line from the station apparatus 100" to the optical distributor-coupler 50' can be achieved readily, if it is tried to construct the transmission lines on the subscribers side with respect to the optical distributor-coupler 50' in dual construction, then optical transmission/reception sections of the optical network units (ONU) 1" to N" must all be constructed in dual construction, and a significant increase in cost cannot be avoided.
Further, if a fault such as a physical damage to an optical transmission line or a failure of an optical reception circuit of an optical network unit (ONU) occurs, then since an upstream optical signal must be transmitted based on a timing of a downstream signal, resultantly an upstream signal cannot be transmitted irrespective of the point of the fault, and there is no available method of discriminating the point of the fault from the station side.
Further, since a small number of optical transmission lines are used commonly by the plurality of optical network units (ONU) 1" to N", if a certain optical network unit (ONU) transmits upstream data at an irregular timing to the station apparatus 100" because of a fault of the certain optical network unit (ONU), then it may possibly interfere with communication between station subscribers of those optical network units (ONU) which are operating regularly.
FIG. 15 illustrates an example of communication interference between station subscribers of the optical network unit (ONU) 1" which is operating regularly. As seen from FIG. 15, since, in the passive optical network (PON) system, a small number of optical transmission lines are used commonly by the plurality of optical network units (ONU) 1" to N", if upstream data to the station apparatus 100" is transmitted at an irregular timing, then there is the possibility that a problem that the upstream data interferes with communication between station subscribers which are operating regularly may occur.
In other words, if, for example, the optical network unit (ONU) 2" transmits an upstream frame at a timing different from the timing at which it should originally be transmitted, then it may interfere with communication of the optical network unit (ONU) 1".
FIG. 16 illustrates an influence of a fault of an optical network unit when the optical network unit transmits an irregular upstream frame. Referring to FIG. 16, by an influence of an upstream frame transmitted from the optical network unit (ONU) 2" with which a fault has occurred, an upstream frame transmitted from the optical network unit (ONU) 1" suffers from frame collision, resulting in failure in communication.
Accordingly, there is a subject to be solved in that it is desirable to secure dual construction of optical transmission/reception sections and so forth of optical network units, specification of a failure fault point in optical transmission lines and optical network units and regular timing transmission of upstream data.